Processes for the recovery of silver in the photographic industry are well known. They can be divided into two broad categories, one being the recovery of silver from spent fixing solution, the other from the photographic emulsion itself. The use of sodium borohydride as a means of reducing the silver halide is also well known. Customarily, it involves the reaction of sodium borohydride with dispersed silver halide at a pH of between 5.5 and 6.5 to precipitate metallic silver, followed by filtration. In all of the above instances, however, high contents of gelatin are either absent or have been converted into some form amenable to filtering or quick precipitation through the use of enzymes, caustics, sodium hypochlorite, or hydrogen peroxide.
Where there are substantial amounts of gelatin present in the system, as for instance when attempting to recover silver from an aqueous dispersion of a silver halide emulsion rather than a fixing solution, the gelatin rapidly blocks the filter. This is particularly true when a filter with fine enough pores is used to prevent silver losses, thus rendering most conventional techniques impractical. Settling tanks, on the other hand, are cumbersome, so that processes involving precipitation are generally space- and time-demanding. This problem is accentuated when recovery is attempted from processor effluent in instances where a wash-off type film is used, i.e., one where development of an image involves washing off the unexposed, unhardened emulsion with a spray of hot water. Typically, water rich in silver halide and gelatin is washed down the sewer at a rate of 5 to 8 gal/minute, often in contravention of local pollution control laws, always with a waste of a precious metal.
The object of the present invention is to provide an inexpensive, easily implemented, continuous process for removing the silver salts from the effluent of a film processor, together with the gelatin, along with a compact apparatus to implement this process.